"What is the restaurant's phone number? I want to book a table."
I'm re-posting an earlier comment of mine from another page that summarizes my understanding of the matter of asking phone numbers.
I'm curious to know, from those of you who are native Mandarin speakers, (1) where you're from, (2) if you're not personally from a Mandarin-speaking area, but speak Mandarin with your family, what region your variety of Mandarin is from, and (3) whether my description is accurate for your region.
- 你的电话（号码）（是）几号？ (This version is common without "是", but it also occurs with it.)
Native Mandarin Chinese speakers have their individual opinions about these, because China is a big place, not to mention the outlying Chinese-speaking countries and regions, and this is a colloquial sentence influenced by local dialects and by quirks of history.
1 is somewhat more northern, but common in a lot of China, and perhaps most common overall. 2 is perhaps half as common as 1, but it's more southern, and common in Singapore, Taiwan, and Malaysia (and perhaps other places, e.g. the southern mainland or some of it), where people will often swear they've never heard 1 (and likewise, proponents of 1 will often swear they've never heard 2). In each of these two cases, proponents of one version will say why the other isn't logical, and none of their reasons is really very convincing. It seems the usage of one or the other is really about history, habit, and experience.
3 is fairly common, but not as common as 1. It feels unnatural to some speakers (particularly to many who like 1) but not to others (those who like 2 are usually fine with it, as are some who like 1). Those who don't like it will even go so far as to say it sounds like you don't really know what a telephone number is, or even what a telephone is.
Where English has a greater influence, 3 seems like the more correct or formal structure to some speakers. That's not to say that 3 doesn't have inherent support by the rules of Chinese itself, but it's not necessarily the go-to phrasing historically in broad swathes of the Mandarin-speaking world, and some native speakers say they never hear anyone use it. But many native Mandarin speakers will swear that it's the only truly correct option.
There are still other ways to ask this question, including shortening the above sentences, but I would stick with those three choices to begin with.
Here are some Youtube videos teaching the "多少" version (which is the most common version on Youtube):
By the way, all of my Taiwanese friends use either 2 or 3, or some variation of those, but, as noted, 1 seems to be popular in YouTube instructional videos by native Chinese speakers (and it's the most common in a Google internet search).